Photodiode array spectrometer

ABSTRACT

A photodiode array spectrometer comprises an arry (40) of photosensitive elements for receiving a beam of light. Light impinging on a photodiode causes discharging of the associated capacitors. The capacitors are recharged periodically by a charge amplifier via a video line (30) by closing transfer switches (SW1, . . . , SW 768) associated with the photosensitive elements, repectively. The switches are group together in several segments which are independently addressable such that during a recharge scan only selected groups of photodioldes are recharged. The information which segments are to be sctivated, i.e. which groups of switches are to be closed, is contained in a segment control block (43). An integration control block (46) additionally permits to adjust the time intervals between successive recharge cycles separately for each selected segment. The invention permits to select regions of interest of the photodiode array for a specific application, whereas other regions are ignored for that application, leading to a reduced data rate with high spectral resolution and sensitivity.

The invention relates to a photodiode spectrometer. Such a spectrometercan be used, for example, for measuring the absorption spectrum of asample substance in order to derive information about the chemicalcomposition of the sample and the quantities of the individualconstituents in the sample.

A photodiode array spectrometer according to the preamble of claim 1 isknown from "A High-Speed Spectrophotometric LC Detector",Hewlett-Packard Journal, Apr. 1984. This known spectrometer is used in aliquid chromatograph for analysing the substances eluting from thechromatographic column. The known spectrometer comprises a light sourceemitting a broad spectrum of ultraviolet and visible radiation and anoptical system for focusing the beam onto a sample cell through whichthe sample substances to be analysed flow. Depending on the specificsubstances flowing through the cell, the sample absorbs certaincharacteristic spectral portions of the radiation entering the samplecell so that the spectral composition of the radiation leaving the cellis indicative of the sample substances.

In the known spectrometer, the spectrum of the radiation leaving thesample cell is derived by means of a diffraction grating arranged in theoptical path behind the cell. The diffraction grating directs light raysof different wavelengths into different directions. A linear array ofphotodiodes is arranged to receive the light diffracted by the grating.Each diode thus receives light corresponding to a different wavelengthrange. The electrical signals produced by the impinging light in eachphotodiode are read out by a read-out circuit and converted to digitaldata values which are representative of the intensity of the lightimpinging on the specific diode. These data values are then displayed asa function of wavelength in any convenient form, for example on a CRTscreen.

The photodiode array is built on semiconductor material and comprises aplurality of photosensitive elements which are connected via electronicswitches to a common output line (video line) which in turn is connectedto a charge amplifier. Each photosensitive element has an associatedcapacitor which represents the junction capacitance of the photodiodes.The combination of photosensitive element and associated capacitor willsubsequently also be referred to as "photocell". Light impinging on thephoto-sensitive material generates charge carriers which discharge thesecapacitors. In operation, the capacitors of the photocells are initiallycharged to a fixed value, respectively, and then the whole array isscanned in predetermined intervals by sequentially closing the switchessuch that the photocells are recharged by the charge amplifier to theiroriginal charge level. The amount of charge transferred hereby causes avoltage change at the output of the charge amplifier which isproportional to the amount of light which has caused the discharge ofthe photocell.

Three important quantities for characterizing the performance of aspectrometer are spectral resolution, spectral range and sensitivity.Spectral resolution indicates how well radiation components with closelyadjacent wavelengths are separated so that they can be identified asseparate components. Spectral range indicates the interval ofwavelengths which can be analysed by the spectrometer. Sensitivityindicates how well weak signals can be distinguished from backgroundnoise and corresponds to the signal/noise ratio. In spectrometers usingphotodiode arrays as light detecting element, the requirement of a widespectral range at a given resolution leads to arrays with a large numberof individual photodiodes, e.g. 1024 diodes as in the knownspectrometer. Due to this large number of photodiodes, the knownspectrometer produces a large number of data values per time unit, i.e.for each reading out of a photosensitive element during a scan, a newdata value is obtained. Consequently, costly data processing and datastorage circuits such as A/D converters, microprocessors, mass storagedevices, are required.

The sensitivity of a spectrometer can generally be improved byincreasing the power of the light source of the spectrometer. As aconsequence thereof, the light power incident on the photodiodes alsoincreases so that the capacitors associated with the photosensitiveelements are discharged to a larger extent than in case of lower lightpower. Since the capacitors must not be completely discharged in orderto avoid nonlinearities and since the size of the capacitors is limitedby economic usage of the chip area of the photodiode array, the scanrate with which the capacitors are recharged has to be increased whenthe light power is increased. Consequently, the number of data valuesper time unit, i.e. the data rate, is increased unless the number ofphotocells is reduced which, however, would result in a smaller spectralrange and/or resolution.

According to the foregoing considerations, known photodiode arrayspectrometers are not satisfactory in all respects because they requirea compromise between spectral resolution, spectral range, sensitivityand data rate so that not all of these parameters can be simultaneouslyselected to the desired value in a specific application. Knownphotodiode array spectrometers therefore require comparatively highcircuit expense if high performance of the spectrometer is desired.

Relative to the prior art, it is an object of the invention to provide aphotodiode array spectrometer which permits increase of spectralresolution and sensitivity without substantially increasing the cost andcomplexity of the signal processing circuitry.

According to an underlying principle of the invention, the wholephotodiode array is subdivided into segments which are selectablyaddressable such that only selected segments of the array are read outduring a scan, whereas the remaining segments are skipped. The segmentor segments of the array which are read out can be selected inaccordance with the specific application, e.g., the wavelength rangewhich is important for the identification and quantitative determinationof the specific sample to be analyzed. Since only the photodiodescorresponding to the relevant spectral range or ranges are read out andthe photodiodes of the remaining spectral range are skipped during aread-out scan, less data are produced within a certain time intervalthan in prior art photodiode arrays of comparable performance whereinduring each scan all the photodiodes are read out. Thus, the data rateof the spectrometer is kept small. On the other hand, the spectrometeraccording to the invention can be used with a higher light power forimproving sensitivity, and with a larger number of photodiodes per unitlength for achieving higher spectral resolution without causing a higherdata rate than in conventional photodiode array spectrometers. Since thespectrometer according to the invention may comprise a large number ofphotodiodes, it can cover a wide spectral range so that many differentsubstances can be analysed which typically have their relevant spectralcharacteristics in different spectral ranges, but yet the data rate canbe kept low because only the photodiodes relevant for the specificapplication are read out during the scans.

According to an embodiment of the invention, the duration of the timeintervals during which the capacitors of the photocells of a specificsegment are discharged (i.e., the integration times) can be individuallyadjusted. For example, the integration times in segments which have lowspectral response, i.e., low incident light intensity, can be selectedgreater than in segments with higher spectral response so that theoverall spectral response is smoothed. In that way, the dynamic range ofthe signals read out from the photodiode array can be reduced so thatthe requirements of the signal processing circuitry such as an A/Dconverter, in particular the resolution requirements thereof, arereduced. Furthermore, the signal/noise ratio can be improved byincreasing the integration time because the strength of the measuringsignal grows proportionally with the integration time, whereas the noiseonly grows according to the square root of the integration timeresulting in an improvement of the signal/noise ratio according to thesquare root of the integration time. In summary, by increasing theintegration times of segments having less spectral response than othersegments, both the dynamic range of the whole spectrum can be smoothedand the signal to noise ratio can be improved.

Subsequently, an embodiment of the invention is explained in more detailwith reference to the drawings.

FIG. 1 is a schematic diagram of a photodiode array spectrometercomprising a photodiode array according to the invention.

FIG. 2 schematically shows a photodiode array of the invention with thecharge amplifier circuit for reading out the array.

FIG. 3 shows in more detail an embodiment of the switch control circuitof a photodiode array according to the invention.

FIG. 1 schematically shows a photodiode array spectrometer which allowsmeasurement of the absorption of a polychromatic beam of ultravioletand/or visible radiation by a sample to be analysed. The spectrometercomprises a light source 1, e.g., a deuterium lamp, which emits a beam 2of polychromatic radiation. The beam 2 is focused by a lens system 3into a sample cell 5. The lens system preferably is an achromatic systemwhich ensures that rays of different wavelengths substantially have thesame focal point. A shutter 4 is provided which permits to interrupt thelight beam 2 in order to measure the dark signal at the photodiodes ofthe photodiode array 11. In the actual measuring process wherein thebeam 2 passes through the sample cell 5, the dark signal and otherelectronic offset signals are subtracted from the measuring values tocompensate for any measuring errors.

The sample cell 5 may comprise an inlet and an outlet through which asample liquid to be analysed flows continuously. Such a spectrometer canbe used, for example, in a liquid chomatograph wherein the inlet isconnected to the chromatographic separation column from which samplesubstances are continuously eluting.

The polychromatic radiation entering the sample cell 5 is partiallyabsorbed by the substances in the cell, whereby, depending on the samplesubstances, rays of certain wavelengths are absorbed more strongly thanrays of other wavelengths. As a result thereof, the beam leaving thecell has a different spectral composition than the beam entering thecell and the resulting spectrum thus contains information about the kindof substances in the cell and about their quantities.

The beam leaving the cell impinges on a holographic diffraction grating10 which disperses the light according to the different wavelengths inthe beam impinging on it. The spatially separated light rays from thegrating 10 impinge on a photodiode array 11, which consists of aplurality of individual light-sensitive diodes 15, 16, etc., which areseparated by light-insensitive gaps. Each of the photodiodes interceptsa specific spectral portion of the diffracted radiation.

The photodiode array 11 is connected to a read-out circuit 20 forperiodically reading out electrical signals from photodiodes, wherebythese signals are indicative of the intensity of the light signalsimpinging on the photodiodes, respectively. Details of the read-outcircuit 20 are explained below with reference to FIGS. 2 and 3. Theelectrical signals read out from the photodiode array 11 are thenfurther processed in a signal processing circuit 21 which typicallycomprises an analog-to-digital converter and circuitry for storing andfurther processing these digital values. The operation of the arrayread-out circuit 20 and the signal processing circuit 21 is controlledby a controller 23, typically comprising a microprocessor, which alsocontrols the operation of a display means 22 for displaying the finalspectrum of the analyzed sample. The signal processing circuit 21 mayalso comprise circuitry for correcting the electrical signals from theindividual photodiodes regarding the above-mentioned dark currents ofthe photodiodes and for other effects.

FIG. 2 schematically shows the photodiode array 11 comprising aplurality of n individual photodiodes 1, . . . , n forming part of asemiconductor chip. Each photodiode has an associated capacitor Cdl, . .. , Cdn which represents the junction capacitance of the photodiodes,or, in applications where a separate capacitor is switched in parallelto the photodiodes, the sum of this capacitor and the junctioncapacitance. The photodiodes and the associated capacitor will also bereferred to as photocells Cell 1, Cell 2, . . . , Cell n. The cells 1 .. . n are connected to a common video line 30. The electricalconnections between the individual cells and the video line can beinterrupted by means of electronic switches SW1 . . . SWn, respectively.The switches are controlled by a switch control circuit 31. The switchcontrol circuit is explained in more detail below with reference to FIG.3.

The video line 30 is connected to a charge amplifier 32 which isdesigned as an integrator comprising an operational amplifier with acapacitor C_(int) in the feedback loop. The non-inverting input of theoperational amplifier is connected to the signal U_(guard) which has afixed potential of, for example, -5 V. Thus, the inverting input (videoline) virtually has the same potential. A reset switch R_(s) across thecapacitor C_(int) is closed before each charge transfer to reset theintegrator.

In operation, the capacitors of selected photocells are initiallycharged to a fixed value. When photons are penetrating thephotosensitive material, charge carriers are generated which dischargethe capacitors corresponding to the amount of photons received within agiven integration period. These capacitors are periodically recharged infurther scan sequences. The discharge level of each individual capacitoris proportional to the incident light intensity during the integrationperiod.

The amount of charge transferred hereby causes a voltage change at theoutput of the charge amplifier 32 which is proportional to the integralof the incident light level during the integration period. The outputsignal of the charge amplifier 32, the "PDA signal", can now be furtherprocessed by additional circuits (not shown), for example, by anamplification circuit, sample and hold circuit, A/D converter andmicroprocessor. Before each charge transfer from the charge amplifier toa cell, the "reset switch" is closed to reset the charge amplifier 32 inpreparation for the next charge transfer.

In the following, an embodiment of the switch control circuit 31 (FIG.2) is explained in detail with reference to FIG. 3. In FIG. 3, the cellsand the associated switches are shown to be arranged in a photocellblock 40. According to an embodiment of the invention, a total number of768 cells is provided arranged in 32 groups or segments with eachsegment comprising 24 cells. Each of these cells is connected via anelectronic switch SW to a common video line 30 connected to the chargeamplifier 32 (FIG. 2).

The switch control circuit is divided into several units, illustrated inFIG. 3 as blocks 41, 43, 44, 45, 46. The photodiode array control unit41 has an input connected via a bus line 42 to a microprocessor (notshown). The output of the photodiode array control unit 41 is connectedto a segment control unit 43 and an integration control unit 46. Thesegment control unit 43 is connected via a plurality of lines En1 . . .En32 to the skip control unit 44 which in turn is connected to a scancontrol unit 45. A plurality of output lines of the scan control unit 45is connected to the integration control unit 46 which has a number ofoutput lines corresponding to the number of electronic transfer switchesSW1 . . . SW768 of the photo cell block 40. Subsequently, the individualunits are described in more detail.

The segment control unit 43 substantially comprises a segment register47 for storing a digital control word having a length of, for example,32 bits. This control word determines the segmentation of the photodiodearray, i.e., which segments are activated during a scan and which arenot. In the present example, with a control word of 32 bits, the arrayis divided into 32 segments and the sequence of "1"s and "0"s in thecontrol word determines which segment is active and which is inactive.The 32 bit control word is initially written into the segment register47 under control of the microprocessor and the photodiode array controlunit 41.

The skip control unit 44 comprises a plurality of logic circuits L1, . .. , L32, the number of which corresponds to the number of output linesEn1, . . . , En32 of the segment register 47. Each logic circuit hasthree input lines, one of which is an output line Eni of the segmentregister (whereby i may be any number between 1 and 32), and two outputlines designated as "Skip" and "Do". Besides the line Eni, the two otherinput lines of a logic circuit Li are the line carrying the "Skip"signal from the previous logic circuit Li-1 and the line carrying the"Ready" signal from the circuit "Shift i-1" which is explained in moredetail below. For the first logic circuit L1, the two input linesbesides the line En1 are connected to the photodiode array control block41 and carry the START-PDA signal.

The scan control unit 45 comprises a plurality of circuits SHIFT 1, . .. , SHIFT 32, the number of which corresponds to the number of segmentsof the photodiode array, i.e., 32 in the present example. In anembodiment of the invention, each of the circuits SHIFT i is a serialshift register having 24 stages, whereby each stage has an outputconnected to a photocell via a logic gate in block 46 (see below). Aninput "In" of a circuit SHIFT i is connected to the output of the logiccircuit Li carrying the "Do" signal. An output "Out" of a circuit SHIFTi is connected to a logic circuit Li+1 for providing a "Ready" signal tothe logic circuit Li+1. Furthermore, each of the circuits SHIFT i isconnected to a line carrying the clock signal supplied by the photodiodearray control block 41.

The integration control unit 46 comprises a number of logic AND gatesG1, . . . , G768 corresponding to the number of photocells. Each ANDgate has two input lines, one of the two being connected to one of the768 output lines of the scan control block 45 and the other beingconnected to a common line 55 designated as "Gate" line which isconnected to a shift register 50, subsequently referred to asintegration shift register. The output signal of each of the AND gatesG1, . . . , G768 is connected to a transfer switch SW1, . . . , SW768,respectively. The integration control unit 46 further comprises arolling register set having a word length of 32 bit according to thenumber of segments into which the photodiode array is divided and adepth of 24 words. At the beginning of each new scan of the array, a newword is loaded into the integration shift register 50 which is later onread out serially via the "Gate" line 55. After the last word has beenloaded into the integration shift register, then, in the next scan, thefirst of the 24 words is again loaded into the shift register.

In the following, the operation of the circuitry shown in FIG. 3 isexplained. The operation of the circuitry is controlled by digitalsignals having two states which are referred to as logic "1" and logic"0". A logic "1" on an output line Eni of the segment register 47 causesthe corresponding circuit Li to generate a "Do" signal which is suppliedto the circuit SHIFT i. As a consequence of the logic "1" at the inputof the circuit SHIFT i, a logic "1" is successively generated on each ofthe 24 output lines of SHIFT i in accordance with the frequency of the"Clock" signal supplied by the photodiode array control unit 41. Inresponse to the logic "1" on the output lines of the circuit SHIFT i andif the "Gate" line 55 carries a logic "1" (enable-signal), thecorresponding transfer switches of the photodiode array are sequentiallyclosed so that the associated photocells are read out. After a logic "1"has successively been applied on all 24 output lines of the circuitSHIFT i, a "Ready" signal is supplied to the logic circuit Li+1 actingas a start signal for the next segment. If a logic "1" is supplied tothe logic circuit Li+1 on line Eni+1, the transfer switches associatedwith the circuit SHIFT i+1 are activated in the manner just described inconnection with the previous segment.

If a logic "0" is applied on a line Enj of the segment register (wherebyj may be any number between 1 and 32), the associated logic cell Ljprovides a "Skip" signal which is directly supplied to the input of thenext logic cell Lj+1. In this case, the segment SHIFT j is skipped andthe associated transfer switches remain opened. If the signal Enj+1 is alogic "1", then a "Do" signal is provided to the segment SHIFT j+1 andthe associated transfer switches are successively activated. If thesignal Enj+1 is a logic "0", then the segment SHIFT j+1 is also skipped.

When the last segment, in the present example the circuit SHIFT 32, hasbeen scanned and has emitted a "Ready" signal or, alternatively, whenthe circuit L32 has emitted a "Skip" signal in response to a logic "0"on line En32, a signal indicating the end of a scan is supplied to thephotodiode array block 41. Thereafter, a new scan can start.

As already mentioned, the activation of the transfer switches isdetermined by the "Gate" signal on the line 55 in addition to thesignals on the output lines of the circuits SHIFT i. The signal on the"Gate" line, either logic "0" or logic "1" is determined by the last bitof the integration shift register 50 (most left position in FIG. 3).During a scan, each time a new segment is started, i.e., when a "Do"signal has been emitted, the contents of the integration shift registeris shifted by one bit such that a new bit is supplied to the "Gate"line. If the bit supplied to the "Gate" line is "0", the signal on thisline acts as a disable signal such that none of the transfer switchesassociated with the presently activated segment is closed, even if anoutput line of the corresponding circuit SHIFT i has a logic "1". If thebit supplied to the "Gate" line is "1", the signal acts as an enablesignal which permits the transfer switches associated with the presentlyactivated segment SHIFT i to be closed provided the signal Eni is logic"1".

During a scan, n of the 32 bits contained in the integration shiftregister are thus successively supplied to the "Gate" line, whereby ncorresponds to the number of selected segments, i.e. the number of logic"1"'s contained in the segment register 47. In the embodiment shownwherein the bits of the integration shift register are shifted to theleft onto the "Gate" line, the left-most bit of the 32 bit word in theshift register enables or disables the first selected segment and then^(th) bit enables the last of the selected segments, i.e. theright-most segment to be activated. The bits n+1 through 32 are notused. The PDA control block 41 takes care that the integration shiftregister 50 is loaded with the content of a row of the rolling registerset in the right order according to the sequence of the selectedsegments. Once a scan is finished, the next 32 bit word is loaded intothe integration shift register. Since there are 24 words, the procedureis repeated 24 times and then it starts again with the first word.

It is important that the total time needed for one scan (scan time) onlydepends on the number of segments selected and the clock rate, whereasthe scan time does not depend on how often the "Gate" signal is enabledor disabled during a scan.

By appropriate selection of the words in the rolling register set, thenumber of times each segment is read out during 24 scans can beadjusted. If, according to a practical example, the numbers indicatinghow often a segment can be read out during 24 scans are 24, 12, 8, 6, 4,3, 2, and 1, the integration times of the segments, i.e. the timesduring which the photocells of a segment are not read, can be adjusted,whereby the available integration times are the following multiples ofthe basic scan time: 1, 2, 3, 4, 6, 8, 12, 24.

According to an embodiment of the invention, a start-up procedure can beimplemented for determining the contents of the rolling register set, inparticular in spectrometric applications. According to this start-upprocedure, the light intensities incident on the various photodiodes ofthe photodiode array are measured without the sample to be analyzedbeing present in the sample cell. Depending on the measured lightintensities, the distribution of "0"'s and "1"'s in the rolling registerset is determined such that segments receiving higher light intensitieswould be read out more often than segments receiving lower lightintensities. Consequently, for segments receiving lower lightintensities there will be more "0"'s in the corresponding column of therolling register set than for segments receiving higher lightintensities.

According to a practical embodiment of the invention, the photodiodearray and associated control circuitry shown in FIG. 3 is used in aspectrophotometer which covers a spectral range from 190 nm to 950 nm.With the 768 photocells, a spectral resolution of 1 nm is achieved.Assuming that only one eighth of the available segments are addressedfor a specific application, i.e., for spectrometric determination of aspecific sample, the data rate can thus be reduced by a factor of 8 ascompared to a conventional self-scanning photodiode array. Furthermore,with a given data rate, the light throughput can be increased by thesame factor which will improve the signal/noise ratio and therefore thesensitivity by a factor of approximately 3. A complete spectrum over thetotal spectral range is acquired in this mode by doing several scanswith different segments.

It is understood that the invention is not limited to an absorptiondetector as described in the embodiment of FIG. 1 but that it can alsobe used, for example, in a fluorescence detector or in an atomicemission detector.

It is furthermore understood that the invention can advantageously beused for analysing the spatial distribution of a beam of radiation,whereby it is not necessary that the spatial components of the beamimpinging on the photodiode array have different wavelengths. Theadvantages of the invention, for example the reduction of the data rate,are also achieved in applications where the light rays impinging ondifferent photodiodes have the same wavelength.

I claim:
 1. Spectrometer for determining the spectral composition of apolychromatic beam of radiation, comprising:an array of photosensitiveelements (1, 2, . . . , n) with each element intercepting a differentwavelength range of the polychromatic beam of radiation, a plurality oftransfer switches (SW1, . . . , SWn), each switch being connected to aphotosensitive element, having a control input terminal for controllingthe opening and closing of the switch, and having an output terminal,and read-out circuitry connected to the transfer switches for opening orclosing the transfer switches and for generating during a read-out cyclesignals indicative of the amount of radiation intercepted by thephotosensitive elements, characterized in that the read-out circuitrycomprises a switch control circuit (41,43,44,45,46) which is designedfor controlling the transfer switches (SW1, . . . , SWn) in selectablegroups (SW1-SW24; SW25-SW48; . . . ) of switches such that during aread-out cycle only selected groups of photosensitive elements (1-24;25-48; . . . n) are read out, with such selected groups comprising lessthan the whole array of photosensitive elements.
 2. Spectrometeraccording to claim 1, further characterized in thatsaid switch controlcircuit comprises a scan control unit (45) having a number n of segmentcircuits (SHIFT 1, . . . , SHIFT n) corresponding to said number n ofphotosensitive elements, each segment circuit (SHIFT 1) comprises aninput line (Do) for transmitting a signal indicative of whether thecorresponding group of photosensitive elements is to be read out or not,and further comprises an output line connected to said control inputterminal of said associated transfer switch of said photosensitiveelement array for controlling the opening or closing of said switches inresponse to the signal on the input line of the segment circuit. 3.Spectrometer according to claim 2, further characterized bya digitalsegment register (47) coupled to said scan control unit (45) for storinga digital word, the number of bits of this word corresponding to thenumber of said groups of said transfer switches and each bit indicatingif said corresponding group of photosensitive elements is to be readout.
 4. Spectrometer according to claim 2, further characterized byanintegration control unit (46) that is coupled to said output lines ofsaid segment circuits (SHIFT 1, . . . , SHIFT n) for the selectableenabling and disabling of the activation of said transfer switches (SW1,. . . SWn).
 5. Spectrometer according to claim 4, further characterizedin thatsaid integration control unit comprises: a rolling register setcontaining a plurality of digital words, the number of bits of each wordcorresponding to the number of said groups of photosensitive elements,an integration shift register (50) having inputs coupled to the rollingregister set for receiving a digital word and an output coupled to agate line (55), and a plurality of AND gates (Gl, . . . , Gn), each ANDgate (Gj) having a first input connected to an output of a segmentcircuit (SHIFT j), a second input connected to the gate line (55), andan output coupled to a transfer switch (SWi).
 6. Spectrometer accordingto claim 3, further characterized byan integration control unit (46)that is coupled to said output of said segment circuits (SHIFTl, . . . ,SHIFTn) for the selectable enabling and disabling of the activation ofsaid transfer switches (SWl, . . . , SWn).
 7. Spectrometer according toany of claims 1-5, or 6,further characterized in that the outputterminals of said transfer switches (SWl, . . . , SWn) are connected toa common video line (30) which is connected to a charge amplifiercircuit for transferring electric charges to a photosensitive elementwhen said associated transfer switch is closed, thus causing a voltagechange at the output of the charge amplifier circuit, the voltage changebeing proportional to the amount of light which has impinged on saidphotosensitive element since the previous opening of said associatedtransfer switch.
 8. Spectrometer according to any of claims 1-5, or 6,further characterized bya radiation source (1) for emitting a beam (2)of radiation, a sample cell (5) for receiving the beam of radiation,with the beam leaving the sample cell being modified by samplesubstances in the sample cell, a wavelength dispersive element forreceiving the beam leaving the sample cell (5) and for generating anoutgoing beam comprising a plurality of spatially separated rays ofdifferent wavelengths which are directed on said photosensitive elementarray (11).